Imagine a world where leather drops, a warrior rolls need, and no one calls ninja. Instead the blacksmith in the group sees gold coins falling before his eyes.
Let's imagine for a moment that armor is built up, not merely hammered into existence fully formed. Mail isn't merely metal, but is built upon a foundation of leather. Similarly, plate as well. This provides a bit more cushioning. And it allows the warrior to roll need.
Imagine if blacksmiths and leatherworkers could add, or remove, metal from armor. Using common materials and common training, they could add on sheets and links of metal, or take them off, allowing far greater flexibility in gear choices.
This brings in some work for LW and BS. It reduces wasted gear. It also gives a slight bias to mail and a higher one to plate. After all, they could just wear the gear without more armor, for no additional cost, while going the other direction requires pulling off the extra metal. They would get some bars from it, but I predict a lot of guilds would provide the upgrading metal by taking the downgrading metal.
Obviously this isn't going to happen. Blizzard has made its armor decisions for Cataclysm. Who knows what will come next, but I doubt a more active role for crafting fits into their vision for WoW.